Rails override attribute using his getter - ruby-on-rails

I have a price attribute in my model.
Can I use attribute-getter, which is named just like the attribute
def price
... logic logic ..
return something
end
in order to override the attribute itself ?
Currently it doesn't work. If I call model.price it works, but when it somes to saving the object via model.save, it stores the default value.
Can it be done in a painless way, or should I make a before_save callback?

If you set a value in Ruby you access the setter method. If you want to override the setter you have to do something like this:
def price=(_price)
# do some logic
write_attribute(:price, _price)
end
This is of course a discussion point. Sometimes you can better use a callback. Something like this:
before_save :format_price
private
def format_price
# Do some logic, for example make it cents.
self.price = price * 100
end

Since you seem to want the "real" value stored in the database, what you probably want to do is modify the setter. This way the actual value is stored, and the price getter can just return it unmodified.
You can do this via the lower level write_attribute method. Something like:
def price=(value)
# logic logic
self.write_attribute(:price, value)
end

If you want to manipulate the attribute's value right before it's saved then using a callback would be a better way, since this is what callbacks are for.

Related

DirtyAttributes take as changed BigDecimal types

I'm in a model callback (after_save) and one of the attributes is BigDecimal type. So when I change another attribute and check dirty attributes with changes method I have this:
{"amount"=>[#<BigDecimal:7f86aa3ac900,'-0.4E3',9(18)>, #<BigDecimal:7f86aa3ac838,'-0.4E3',9(18)>], "description"=>["vvvv", "ccc"]}
It instantiates amount as BigDecimal and takes object_id as part of the changes.
Has anyone an idea of how to avoid this behaviour?
If in after_save you need to check if a particular BigDecimal field is really changed, you need to reload rails-created method attr_name_changed? (in your case amount_changed?):
def amount_changed?
if amount_change.present?
amount_change[0].to_f != amount_change[1].to_f
end
end
What it does it compares before (amount_change[0]) and after (amount_change[1]) values in float form.
So then in after_save callback you can do:
after_save :do_something_if_amount_changed
def do_something_if_amount_changed
if amount_changed?
do_something
end
end

Is it a correct / not dangerous / common approach to pass an 'ActiveRecord::Relation' object as a method parameter?

I am using Ruby on Rails 3.2.2 and I would like to know if it is a correct / not dangerous / common approach to pass an ActiveRecord::Relation object as a method parameter.
At this time I am planning to use this approach in a scope method of a my model this way:
class Article < ActiveRecord::Base
def self.with_active_associations(associations, active = nil)
# associations.class
# => ActiveRecord::Relation
case active
when nil
scoped
when 'active'
with_ids(associations.pluck(:associated_id))
when 'not_active'
...
else
...
end
end
end
Note I: I would like to use this approach for performance reasons since the ActiveRecord::Relation is lazy loaded (in my case, if the active parameter value is not active the database is not hit at all).
Note II: the usage of the pluck method may generate an error if I pass as association parameter value an Array instead of an ActiveRecord::Relation.
1) In my opinion it's a sound tradeoff, you lose the ability to send an array as argument but you gain some perfomance. It's not that strange; for example, every time you define a scope you are doing exactly that, a filter than works only on relations and not on arrays.
2) You can always add Enumerable#pluck so the method works transparently with arrays. Of course it won't work if you use more features of relations.
module Enumerable
def pluck(method, *args)
map { |x| x.send(method, *args) }
end
end

Rails: Initializing attributes that are dependent on one another

I have the following classes in my ActiveRecord model:
def Property < ActiveRecord::Base
# attribute: value_type (can hold values like :integer, :string)
end
def PropertyValue < ActiveRecord::Base
belongs_to property
# attribute: string_value
# attribute: integer_value
end
A PropertyValue object is intended to hold only a string value or an integer value, depending on the type, specified in the value_type attribute of the associated Property object. Obviously, we shouldn't bother the user of the PropertyValue class with this underlying string_value/integer_value mechanism. So I'd like to use a virtual attribute "value" on PropertyValue, that does something like this:
def value
unless property.nil? || property.value_type.nil?
read_attribute((property.value_type.to_s + "_value").to_sym)
end
end
def value=(v)
unless property.nil? || property.value_type.nil?
write_attribute((property.value_type.to_s + "_value").to_sym, v)
end
end
I want to offer the user a view to fill in a bunch of property values, and when the view is posted, I'd like to have PropertyValue objects instantiated based on the list of attributes that is passed in from the view. I'm used to using the build(attributes) operation for this. However, the problem now occurs that I don't have any control over the order in which the attribute initialization takes place. Thus the assignment of the value attribute will not work when the association with the Property attribute has not yet been made, since the value_type cannot be determined. What is the correct "Rails" way to deal with this?
BTW, as a workaround I have tried the following:
def value=(v)
if property.nil? || property.value_type.nil?
#temp_value = v
else
write_attribute((property.value_type.to_s + "_value").to_sym, v)
end
end
def after_initialize
value = #temp_value
end
Apart from the fact that I think this is quite an ugly solution, it doesn't actually work with the "build" operation. The #temp_value gets set in the "value=(v)" operation. Also, the "after_initialize" in executed. But, the "value = #temp_value" does not call the "value=(v)" operation strangely enough! So I'm really stuck.
EDIT: build code
I indeed realized that the code to build the Property objects would be handy. I'm doing that from a Product class, that has a has_many association with Property. The code then looks like this:
def property_value_attributes=(property_value_attributes)
property_value_attributes.each do |attributes|
product_property_values.build(attributes)
end
end
At the same time I figured out what I did wrong in the after_initialize operation; it should read:
def after_initialize
#value = #temp_value
end
The other problem is that the property association on the newly built property_value object will never be set until the actual save() takes place, which is after the "after_initialize". I got this to work by adding the value_type of the respective property object to the view and then having it passed in through the attributes set upon post. That way I don't have to instantiate a Property object just to fetch the value_type. Drawback: I need a redundant "value_type" accessor on the PropertyValue class.
So it works, but I'm still very interested in if there's a cleaner way to do this. One other way is to make sure the property object is attached first to the new PropertyValue before initializing it with the other attributes, but then the mechanism is leaked into the "client object", which not too clean either.
I would expect some sort of way to override the initializer functionality in such a way that I could affect the order in which attributes get assigned. Something very common in languages like C# or Java. But in Rails...?
One option is to save the Property objects first, and then add the PropertyValue objects afterwards. If you need to you could wrap the whole thing in a transaction to ensure that the Properties are rolled back if their corresponding PropertyValues could not be saved.
I don't know what your collected data from the form looks like, but assuming it looks like the following:
#to_create = { :integer => 3, :string => "hello", :string => "world" }
You could do something like this:
Property.transaction do
#to_create.keys.each do |key|
p = Properties.create( :value_type => key.to_s )
p.save
pval = p.property_value.build( :value => #to_create[key] )
pval.save
end
end
That way you don't have to worry about the nil check for Property or Property.value_type.
As a side note, are you sure you need to be doing all this in the first place? Most database designs I've seen that have this kind of really generic meta-information end up being highly non-scalable and are almost always the wrong solution to the problem. It will require a lot of joins to get a relatively simple set of information.
Suppose you have a parent class Foo that holds the property/value pairs. If Foo has ten properties, that requires 20 joins. That's a lot of DB overhead.
Unless you actually need to run SQL queries against PropertyValues (e.g. "get all Foos that have the property "bar"), you could probably simplify this a lot by just adding an attribute called "properties" to Foo, then serializing your Properties hash and putting it in that field. This will simplify your code, your database design, and speed up your application as well.
Oh jeeezzzz... this is insanely simple, now that I puzzled on it a little more. I just need to override the "initialize(attributes = {})" method on the PropertyValue class like so:
def initialize(attributes = {})
property = Property.find(attributes[:property_id]) unless attributes[:property_id].blank?
super(attributes)
end
Now I'm always sure that the property association is filled before the other attributes are set. I just didn't realize soon enough that Rails' "build(attributes = {})" and "create(attributes = {})" operations eventually boil down to "new(attributes = {})".
Probably you should try to use ActiveRecord get/set methods, i.e.:
def value
send("#{property.value_type}_value") unless property || property.value_type
end
def value=(v)
send("#{property.value_type}_value=", value) unless property || property.value_type
end

custom attr_reader in rails

Mostly in rails if you write my_obj.attr it looks up attr in the database and reports it back. How do you create a custom def attr method that internally queries the database for attr, modifies it and returns? In other words, what's the missing piece here:
# Within a model. Basic attr_reader method, will later modify the body.
def attr
get_from_database :attr # <-- how do I get the value of attr from the db?
end
Something like that:
def attr
value = read_attribute :attr
value = modify(value)
write_attribute :attr, value
save
value
end
Neutrino's method is good if you want to save a modified value back to the database each time you get the attribute. This not recommended since it will execute an extra database query every time you try to read the attribute even if it has not changed.
If you simply want to modify the attribute (such as capitalizing it for example) you can just do the following:
def attr
return read_attribute(:attr).capitalize #(or whatever method you wish to apply to the value)
end

How can all ActiveRecord attribute accessors be wrapped

I've got a model in which attributes are allowed to be null, but when a null attribute is read I'd like to take a special action. In particular, I'd like to throw a certain exception. That is, something like
class MyModel < ActiveRecord::Base
def anAttr
read_attribute(:anAttr) or raise MyException(:anAttr)
end
end
that's all fine, but it means I have to hand-code the identical custom accessor for each attribute.
I thought I could override read_attribute, but my overridden read_attribute is never called.
Not sure why you'd need to do this, but alas:
def get(attr)
val = self.send(attr)
raise MyException unless val
val
end
#object.get(:name)
That's funny, we were looking into this same thing today. Check into attribute_method.rb which is where all the Rails logic for the attributes exists. You'll see a define_attribute_methods method which you should be able to override.
In the end, I think we're going to do this in a different way, but it was a helpful exercise.

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